1. Field of the Invention
The present invention relates to a semiconductor laser optical source used as an optical source for laser beam printers or optical disk devices, and more particularly to a short wavelength semiconductor laser device formed with a semiconductor of a forbidden band width of 2 eV or more.
2. Description of the Related Art
Conventional short wavelength semiconductor laser devices have been formed of semiconductor materials of GaN and AlGN or the like with wide forbidden band width of more than 2 eV. For the materials of such wide forbidden band width, it has been known that either one of p-type doping or n-type doping is extremely difficult to attain a semiconductor device using the pn junction. Also, for the semiconductor material such as GaN or AlGaN, it has long been considered to realize a semiconductor laser device since p type impurity doping is particularly difficult.
With the progress in crystal growth techniques in recent years, p type doping has become possible even for the semiconductor materials described above, and semiconductor lasers have been developed for practical use. The p type doping, however, is not still easy for the type of the materials described above. For this case, there has been known, for example, that a stacked structure as shown in FIG. 17 for obtaining p type impurity density necessary for practical semiconductor laser devices. To obtain the p type impurity, a p-cladding layer is formed as a super lattice structure of GaN having a relatively narrow forbidden band width and Al0.16Ga0.84N having a relatively wide forbidden band width, and the energy of p type impurity doped to the Al0.16Ga0.84N layer is at a level lower than the Fermi level for the entire super lattice. However, in the ridge waveguide semiconductor laser device, since a current path is only a path in which a current vertically flows in the ridge having a rectangular cross section, the device resistance could not be reduced significantly even with considerations.
One example of the semiconductor laser structures manufactured by using the technique described above includes the document of T. Mizuno, et al. (19th IEEE International Semiconductor Conference 63p).
On the contrary, as a current confinement structure of a semiconductor laser device, a structure referred to as a self-aligned structure is known, which attains the current confinement and the optical waveguide simultaneously with a groove formed in a current blocking layer as described above. Also, for nitride type semiconductor laser device, a structure shown in FIG. 18 has been disclosed (JP-A No. 2003-179314) by Nakajima, et al, for example. In the figure, reference numeral 1 denotes a semiconductor substrate; 7, n-cladding layer; 8, an active layer; 9, a p-cladding layer; 4, a current blocking layer; and 1, a single composition n-cladding layer (re-growth cladding layer). In this structure, the narrowest portion in the current blocking layer 4 is positioned several hundreds nm away from the activate layer 8 to provide a current distribution depicted by solid lines with reference numeral 12, over which current flows.